%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-7-18 on page II-205 in "Horizontal spaces".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{111.0pt}

  % Shorten the vertical size of the example ...
  \AtBeginDocument{%
    \setlength\abovedisplayskip{.5\abovedisplayskip}%
    \setlength\belowdisplayskip{.5\belowdisplayskip}%
  }

%StartShownPreambleCommands
\usepackage[fleqn]{amsmath}
%StopShownPreambleCommands

\begin{document}
\begin{gather}
  X \,  a \enspace b \quad c \qquad d                     \\
  X_{y_{a \enspace b \quad c \qquad d}}                   \\
  X_{y_{a \mspace{9mu} b \mspace{18mu} c \mspace{36mu} d}}
\end{gather}
\end{document}
